1. Field of the Invention
The present invention relates to the measurement of heart motion employing magnetic resonance imaging and, more specifically, it relates to a process of employing harmonic phase images acquired using magnetic resonance imaging in order to track material points and calculate Lagrangian strain in the heart.
2. Description of the Prior Art
The advantageous use of magnetic resonance imaging wherein a patient or object is placed within a magnetic field with alternating generation of RF pulses and gradient pulses serving to excite nuclei within the area of interest and cause responsive emission of magnetic energy which is picked up by a receiver and may be processed by computer means followed by recording, display or production of hard copy of the results has long been known. See, generally, Atalar-McVeigh U.S. Pat. No. 5,512,825 and Conturo-Robinson U.S. Pat. No. 5,281,914, both of which are assigned to the owner of the present invention, the disclosures of which are expressly incorporated herein by reference.
It has been known to employ two sets of tagging planes oriented orthogonal to the image plane in imaging two-dimensional heart wall motion with magnetic resonance imaging through spatial modulation of magnetization (SPAMM) approaches. See U.S. Pat. Nos. 5,054,489, 5,111,820 and 5,217,016. See also, Axel et al., “MR Imaging of Motion with Spatial Modulation of Magnetization,” Radiology, 171:841-845, 1989 and Axel et al., “Heart Wall Motion: Improved Method of Spatial Modulation of Magnetization for MR Imaging,” Radiology, 172(1):349-350, 1989.
It has been known in connection with magnetic resonance tagging to employ image processing techniques to detect tag features and subsequently combine the features into a detailed motion map related to displacement and strain with subsequent interpolation being employed. See, for example, Young et al., “Three-Dimensional Motion and Deformation with Spatial Modulation of Magnetization,” Radiology, 185:241-247, 1992 and McVeigh et al., “Noninvasive Measurements of Transmural Gradients in Myocardial Strain with MR Imaging,” Radiology, 180(3):677-683, 1991. These approaches are not automated, as they require some manual intervention.
It has also been known to employ optical flow methods in respect of magnetic resonance tagging image sequences. See, generally, Prince et al., “Motion Estimation from Tagged MR Image Sequences,” IEEE Trans. on Medical Imaging, 11(2):238-249, June 1992; Gupta et al., “On Variable Brightness Optical Flow for Tagged MRI,” Technical Report, 95-13, JHU/ECE, 1995; and Gupta et al., “Bandpass Optical Flow for Tagged MR Imaging,” Proceedings of the IEEE International Conf. on Image Proc., Santa Barbara, 1997. In such approaches sinusoidal tag patterns are employed instead of saturated planes. Image brightness gradients are features, which together with temporal derivatives estimated from image pairs, can be used to provide dense motion estimates generally referred to as “optical flow.” Such approaches require regularization to compensate for the fact that the brightness gradients contain information about motion solely in the direction of the gradient.
U.S. Pat. No. 5,275,163 discloses the use of magnetic resonance imaging in monitoring motion of a part of an object. Pulse and gradient sequences are applied in pairs with spatially differing tagging patterns and subtraction being employed to form a tagged image.
U.S. Pat. No. 5,352,979 discloses observing a phase angle response of volume elements in a slice or volume and imaging occurring before and during perturbations caused by external stimuli.
U.S. Pat. No. 5,379,766 discloses quantitative motion evaluation of a portion of an object by employing a high contrast-tagging grid for detection of tagging patterns.
U.S. Pat. Nos. 5,315,248 and 5,545,993 disclose tracking of motion.
It has been known to employ planar tag analysis in magnetic resonance imaging. It has also been known to employ such approaches in connection with the analysis of myocardinal motion. Such prior art methods typically involve extraction of motion from these images through displacement vectors or strain patterns and involves tag identification and position estimation followed by interpolation.
Phase contrast magnetic resonance imaging has also been known. It provides a method for directly measuring motion by measuring a property sensitive to velocity and reconstructing velocity fields with strain being computed by employing finite differences. One of the problems with these two approaches is that planar tagging images cannot be accurately analyzed automatically. Phase contrast images, while capable of being analyzed automatically, tend to have a low signal-to-noise ratio leading to unacceptable results.
It has been known that strain measurements in the heart muscle can be significant in the diagnosis and quantification of heart disease. Developments over the past decade in tagged cardiac magnetic resonance imaging have made it possible to measure the detailed strain patterns of the myocardium in the in vivo heart. MR tagging employs a special pulse sequence to spatially modulate the longitudinal magnetization of the subject to create temporary features referred to as “tags” in the myocardium. Tagged MRI has been employed to develop and refine models of normal and abnormal myocardial motion, to better understand the correlation of coronary artery disease with myocardial motion abnormalities, to analyze cardiac activation patterns using pacemakers to understand the effects of treatment of myocardial infarction and in combination with stress testing for the early detection of myocardial eschemia. In spite of the successful scientific efforts, tagged MRI has been slow to enter into routine clinical use because of long imaging and post processing times, inadequate access to patients during imaging and lack of understanding of MR processing benefits by clinicians and their associates.